Add gunicorn timeout

[matherit]

Issue #, if available:

Description of changes: Makes the GUnicorn startup timeout configurable via an environment variable. I also updated the readme to include a section that details configurable env vars like this one that do not fit under current sections.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sagemaker-bot]

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